In vitro Study of Assumed in vivo Chiral Conversion of Clenbuterol

Abstract

In order to explore the possibility of determining whether clenbuterol (CLB) was ingested unintentionally via meat products contaminated with CLB or taken intentionally for doping purposes by athletes, we conducted an in vitro study assuming in vivo chiral conversion of CLB. Enzymatic reaction using swine liver tissue or chemical reaction in artificial gastric juice was performed to clarify the chiral conversion of CLB enantiomers. LC/UV measurement revealed no chiral conversion in the enzymatic reaction and temperature-dependent chiral conversion in the artificial gastric juice where CLB finally racemized. From the calculated reaction rate, activation energy (Ea), and activation entropy (ΔS) in the chiral conversion reaction of R-CLB in artificial gastric juice, we expected that this chiral conversion would proceed slowly because Ea was relatively high. After heating at 38°C for 2 h, only approximately 1% of CLB underwent chiral conversion. Therefore, when humans ingest meat products contaminated with CLB having a different enantiomeric ratio, chiral conversion hardly progresses in the stomach and such ingestion would have very little effect on the enantiomeric excess of CLB excreted in urine. This suggests that measuring urinary CLB enantiomeric ratio would reveal whether CLB was ingested unintentionally via CLB-contaminated meat products or taken intentionally.